The present invention relates to a method for recovering lecithin from mucilaginous substances obtained in the production of vegetable fats and oils by extraction of the mucilaginous substances with a solvent which is gaseous under normal conditions (0.degree. C. 1 bar).
Lecithin is a mixture of various phosphatides belonging, in particular, to the group of lecithins and cephalins. Phosphatides are fat-like triglycerides which are soluble in hydrocarbons and insoluble in acetone, in which the phosphoric acid replaces a fatty acid radical and is esterified with the OH group of choline, colamine, serine, inositol or another serine-like base. Since the phosphatides contain lipophilic and hydrophilic as well as acid and basic groups in one molecule, lecithin is used particularly as an emulsifier. Additionally, lecithin is used as a feed additive as well as for the manufacture of cosmetic and pharmaceutical preparations.
Phosphatides are natural substances which occur together with animal and vegetable fats, e.g., in egg yolks and in soy beans. Raw vegetable oils and fats obtained by melting, pressing and/or extraction of fruits and seeds, in particular, contain more or less phosphatides which are obtained as mucilaginous substances by treating the raw vegetable oils and fats, after they have been freed from solids, with water, water vapor and/or diluted acids. These mucilaginous substances are centrifuged out and contain, in addition to the phosphatides, accompanying substances which are oils or fats, fatty acids, bittering and dyestuffs. In a purification process, the mucilaginous substances, also called raw lecithin, are processed into lecithin which can then be used for the production, processing and conservation of food as well as pharmaceutical and cosmetic preparations.
U.S. Pat. No. 4,367,178 discloses a process for recovering lecithin from raw lecithin in which the raw lecithin is treated in an extraction stage with a gas which is in the supercritical state with respect to its pressure and its temperature to form an extract containing supercritical gas phase and an extraction residue comprised of purified lecithin. The extract-containing supercritical gas phase is then conducted from the extraction stage to a separator stage where a separation into gas and extract is effected by way of changes in pressure and/or temperature. The gas is returned to the extraction stage. The purified lecithin which remains as a residue in the extraction stage is removed from the extraction stage either continuously or discontinuously. In this process, gaseous halogenated hydrocarbons, as well as the gases CO.sub.2, SF.sub.6, N.sub.2 O, SO.sub.2, C.sub.2 H.sub.6, C.sub.2 H.sub.4, C.sub.3 H.sub.8 and/or C.sub.3 H.sub.6 are used as extraction agents. If CO.sub.2 is used as the extraction agent, the extraction stage operates at a pressure of 72 to 800 bar and at a temperature of 31.3.degree. to 100.degree. C., and the separation stage operates at a pressure of 10 to 72 bar and at a temperature of 10.degree. to 32.degree. C. The extract separated from the supercritical gas phase includes the accompanying substances contained in the raw lecithin, particularly fats and oils, respectively, and water. The process disclosed in U.S. Pat. No. 4,367,178 has the drawback that the extraction time, which is preferably 3 to 7 hours, is rather long, which is caused by the poor solubility in the supercritical gas phase of the substances accompanying the raw lecithin.